Among plant pathologists there is to-day no topic of more engaging interest and no problem more difficult than that of the nature of the causal agency in mosaic and allied plant diseases. Possibly we might extend this statement so as to comprehend at the same time the causal factors involved in those types of vegetative variegation, or modified pigmentation, whether infectious or not, which afford the decorative mottled, spotted, and striped plants so much cultivated for foliage effects.At various times almost every conceivable view has been held as to the nature of the etiological agent in tobacco mosaic, but from the earliest experiments it has been perfectly clear that the disease is transmissible. The great majority of workers have accepted the evidence of the filterable character of the infective agency, and it is this which gives to the disease much of its peculiar interest.The relation of the true mosaic diseases to certain other types of 1 This paper was read at the annual meeting of the American Philosophical Society,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.